1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of Related Art
A steering gearbox of an electric power steering system described in Japanese Patent Application Publication No. 10-258749 (JP 10-258749 A) includes a housing, a rack shaft, a pinion shaft, and an assist force transmitting mechanism. The assist force transmitting mechanism transmits an assist force from an electric motor to the pinion shaft via a worm gear and a worm wheel. The housing includes a rack housing, a pinion housing, and a junction housing. The rack housing accommodates the rack shaft. The pinion housing accommodates the pinion shaft and the assist force transmitting mechanism. The junction housing accommodates a meshing region at which a pinion gear of the pinion shaft and a rack gear of the rack shaft are meshed with each other. An upper portion of the junction housing is covered with the pinion housing. The junction housing has an opening portion that is communicated with the space inside the pinion housing from below.
End portions of the rack shaft respectively protrude from open ends of the rack housing and are connected to tie rods. In order to make the housing water-tight and dust-tight, junctions between the end portions of the rack shaft and the tie rods are respectively covered with boots. An annular oil seal is press-fitted into the opening portion of the junction housing. The oil seal closes an annular clearance between the opening portion and the pinion shaft. With this configuration, even if foreign matter, such as muddy water, enters the rack housing from the open end of the rack housing due to damage to the boot, the oil seal prevents the foreign matter from entering the pinion housing. It is thus possible to protect a speed-reducer, such as the assist force transmitting mechanism, and electric components, such as a motor and an electronic control unit (ECU), disposed in the pinion housing.
In order to assemble the electric power steering system described in JP 10-258749 A, the pinion gear of the pinion shaft is first inserted into the pinion housing, is then passed through the oil seal, and is then meshed with the rack gear in the junction housing. If the pinion gear is covered with a jig, the pinion gear is prevented from causing damage to the oil seal while the pinion gear is passing through the oil seal.
However, in the electric power steering system described in JP 10-258749 A, the oil seal is disposed in the junction housing. Due to this configuration, the pinion gear that has been passed through the oil seal and accommodated in the junction housing is not externally exposed. Therefore, the jig cannot be detached from the pinion gear, and thus the jig cannot be used. As a result, the pinion gear may cause damage to the oil seal while the pinion gear is passing through the oil seal. If a pinion gear having a small diameter is employed in order to prevent the pinion gear from causing damage to the oil seal, it is not necessary to use a jig. However, the diameter of a portion of the pinion shaft that is fitted into the oil seal increases, and thus the weight of the pinion shaft increases.